Power Former ~ Project

ABSTRACT :
This paper provides an overview on the aspects of the powerformer . VOLTAGE COLLAPS issues have, in recent years begun to pose an undesirable threat to the operational security of power systems. Recent collapses, including the 1996 collapse of the western U.S. grid, have highlighted the importance of avoiding generator limiting in order to limit potential voltage instability. The particular importance of the stator current limitation and its contribution to the collapse of a system has also been highlighted. The focus of this paper is a new type of generator, the Powerformer, which connects directly to the high voltage bus, and therefore, controls this high side bus’s voltage directly. The step-up transformer imposes great drawbacks on the power plant as a whole, starting from reduction in efficiency, high maintenance costs, and more space, less availability and not to forget the increased environmental impact of the plant. It also highlights the use of over excitation limiters and its classification. The simulation models, the advantages in various aspects, the tabular comparisons with conventional generator are been discussed regarding power former.

CONTENTS:
INTRODUCTION
POWERFORMER CONCEPT
STATOR CORE DESIGN
ADVANTAGES
OVER EXCITATION LIMITERS
TABULAR COMPARISIONS
CONCLUSION
REFERENCES




1. INTRODUCTION:
Today’s high-voltage generators are constructed in such a way that limits their output voltage to a maximum of 30 kV. The power grid with voltages up to 800 kV, can not be directly supplied by those generators, a reason why large power plants nowadays are using power step-up transformers in order to transform their generated voltage to a higher voltage level suitable for the interface with the transmission grid. The step-up transformer imposes great drawbacks on the power plant as a whole, starting from reduction in efficiency, high maintenance costs, more space, less availability and the increased environmental impact of the plant.
During the last century, a number of attempts were made at developing a high-voltage generator that could be connected directly to the power grid, i.e. without going via the step-up transformer. However, although grid voltages can reach 800 kV or more, generators are presently constructed for voltages up to only 30 kV, as stated above. ABB has developed in close cooperation with Vattenfall (the Swedish state-owned power utility) a new high voltage generator with innovative features that enables it to be connected directly to the transmission grid; its output voltage can reach levels up to 400 kV. With the new technology, future transformer-less power plants can be constructed leading to a new concept of energy systems. The new machine has been named Powerformer; its benefits such as higher efficiency, better availability, lower maintenance costs and reduced environmental impact are straightforward consequences of transformer-less power plants.
2. POWERFORMER CONCEPT:
Powerformer, although a new machine is a 3-phase AC generator with a rotor of conventional design. The difference, compared with conventional generator, lies in the stator windings. In Powerformer stator winding consists of high-voltage cables instead of today’s windings with a square cross-section. By using high-voltage cables as generator stator winding, it is possible to highly increase the generated voltage. The decisive difference between this design and present-day technology is that Powerformer allows direct connection to the high-voltage grid. This is illustrated in Figure (1).


Figure (1):
1. Generator.
2. Generator circuit breaker.
3. Surge arrester.
4. Step-up transformer
5. Line circuit breaker
(a). When the same plant when the new technology is used.
(b). This design implies the omission of the generator circuit-breaker, the high current bus-bar and the step-up power transformer from the power plant, because Powerformer comprises the functions of both generator and step-up transformer as realized from Figure (1). As a consequence there is an increase up to 1.5% in total electric power efficiency compared with today’s best designs, without using superconductive materials. Reactive power output and overload capability are also improved. There are also major changes in design, construction, manufacturing and production of the total plant. These give a total reduction in size and weight, thereby giving less environmental impact. Other advantages achieved by the utilization of Powerformer will be discussed shortly in more details.The technological bases of the new machine gives a promising future possibility for both hydro and thermo power plants as will as for other electrical equipment.
3. STATOR CORE DESIGN :
                 Powerformer windings: The magnetic circuit of Powerformer makes certain demands on the inding.The winding consists of a power cable with solid insulation and two semiconducting layers, one surrounds the conductor and the other outside the insulation, the semiconducting layers serves as an equi-potential surface that forces the electric field to be uniform around the circumference. The insulationforces the electric field to be uniform around the circumference. The insulamaterial is cross-linked polyethylene (XLPE) used in high-voltage power cables. On the other hand the circular geometry of power former windings provides an evenly distributed electric field. This means that the insulation material will be uniformly stressed and utilized in an optimum way.
Secondly, powerformer cable is designed for an electric field stress of 10 kV/mm, as compared to the 3 kV/mm, that today’s generator windings.  


4. ADVANTAGES OF POWERFORMER:
             Powerformer concept results in major changes in the design, manufacturing and construction of a complete power plant along with its operation, these changes can a improve the economy of the whole power plant. The ability of Powerformer to replace the generator and step-up power transformer used in today’s power plants, and produce a generated high voltage level, that enables the direct connection to the transmission power grid (see Figure (1)) has many benefits to the power plant  whole.
4.1 Efficiency:
             In general, a power plant Powerformer has 0.5-1.5 % higher efficiency than a conventional power plant (Powerformer plants have 0.5-1.5% less active losses than conventional plants). For a 120 MW plant, the figure is about 1.5%. This means that a plant with Powerformer will produce 1.8 MW more power than a conventional plant. This extra power, obviously, improves the economy of the whole plant
4.2 Reactive Power Capability:                                                                                                  
     The generation of reactive power is needed to compensate for the reactive power losses in the transmission networks. With Powerformer the reactive power losses in the step-up  transformer is eliminated. With more reactive power capability, Powerformer  will become competitive alternative to traditional Reactive Power compensators (RPC), because Powerformer, unlike traditional RPCs can be overloaded over rather long period of time, due to its robust design. This feature is desired during disturbances in high voltage transmission networks. For example neither the copper windings nor the laminated core in Powerformer are affected by any rapid temperatures, this considerably reduces the risk of damage to the generator insulation.
4.3 Maintenance and Availability:
Powerformer-based plants are simpler to operate with substantially smaller number of components than their conventional counterparts, a realistic situation for hydro-electric power station is provided.

1. Generator hall
2. Generator
3. Bus-bars system
4. Tunnel system
5. Transformer bay
               For example Powerformer technology eliminates the step-up transformer, the handling of oils, generator circuit breaker and part of the bus-bar system. Fewer components mean fewer sources of potential faults and thus considerably lower maintenance and maintenance costs. With Powerformer, the availability of the power plant will be improved, because again fewer components mean fewer faults and more availability. Powerformer operates with high voltage and low current, the heat developed in its stator windings will, therefore, be lower than in conventional generator stator windings having the same ratings. The stator of Powerformer will thus operate at lower temperature so that the materials making it are less stressed, which leads to fewer faults and higher availability. By comparing statistical availability figures for conventional power plants with the expected availability of a plant based on Powerformer, assuming a nominal operation period of 7000 hours per annum, it has been found that a plant with Powerformer will have 1.0 % higher availability, this equals 70 hours of operation during which a Powerformer based-plant will keep on producing power and income while a conventional plant is out of operation (unavailable).
4.4 Environmental Impact:  
A Life-Cycle Assessment (LCA) is a tool to provide an overall picture of the environmental impact from a product or a system through out its lifetime from raw material extraction, production, use, recycling, and finally to disposal. LCA has been performed on two systems both connected to a 130 kV power grid, a 150 MVA Powerformer versus a conventional 136 MVA generator, breaker and transformer system.A lifetime of 30 years was assumed, the environmental impact is expressed in Environmental Load Unit (ELU), a high impact on the environment gives large ELU number. The results are shown in Figure (6), which shows that the Powerformer system has lower environmental impact than a traditional system during all of its life time phases, this is mainly because Powerformer has less energy losses. Results from a comparative LCA study on Powerformer system and a conventional generator system. Powerformer is clean and safe; a conventional step-up transformer contains several tones of oil. The handling of the oil-based insulation and cooling systems with the associated fire and leakage risks are avoided, giving a cleaner and safe power plant. Powerformer fully insulated winding minimizes the risk of partial discharge, hence, less risk of ozone production and more environmentally friendly power plants. Finally, much of the material used in Powerformer can be easily recycled after the decommission of the machine, a matter that has been considered with concern in the design of Powerformer and in the material used.

5. OVEREXCITATION LIMITERS:
 The purpose of overexcitation limiters is to ensure that the generator windings are not damaged due to heating caused by excessive current flows. Overexcitation limiters can be installed to limit the currents in both the rotor and stator windings. The interaction between rotor and stator limiters is extremely important in the context of system stability and it has been pointed out that rotor limited generator, subject to decreasing voltages can become stator limited.It has also been pointed out that armature current limiter affects the power system in a more drastic manner than does a rotor current limiter. Delaying stator current limitation will therefore be extremely beneficial. While it is common to have a rotor overexcitation limiter installed, in a majority of cases, the limiting function on the stator windings is performed by an over current relay. The over current relay instantaneously disconnects the generator from the grid if the stator current becomes excessive. Clearly the case where the relay operates will be more extreme than that of the limiter as it removes the machine completely from service. Although the majority of generators are protected by overcurrent relays, there are a number of cases where a current limiter, rather than a relay protects the stator current .The overexcitation limiters are used to protect the stator currents at numerous nuclear power plants in Sweden. With this in mind and remembering that the topic of this paper is the PowerformerS, its ability to maintain a higher stator current overload capability and the impact of the resultant higher limit level is therefore of particular interest and should be suitably modeled.  Comparison between conventional and power former armature overload capability curves:







6. TABULAR COMPARISIONS: The above tabular comparisons clearly show the superiority of Powerformer based-plans over conventional plants.
7. CONCLUSION:
The power transformer (a high voltage generator), has been studied through out this paper; the new concept provides the possibility to directly connect a rotating machine to the high-voltage power grid without going via a step-up transformer. Powerformer is a high-voltage generator;  the limitation in its rated voltage is set solely by the AC power cable and the cable accessories (terminations and joints) used in its stator winding. This means that Powerformer of voltage ratings up to 400 kV can be realized, although has not been proved yet. Most important is that the over load capability of the powerformer can have more beneficial impact on efficiency of power plant.
8. REFERENCES:
[1] C.W. Taylor, “Improving grid behavior,” IEEE Spectr., vol. 36, pp. 40– 5, 1999.
[2] F. G. A. Sjogren, S. G. Johansson, and J. E. Daalder, “Behavior of generator current limiters near the point of voltage collapse,” in Proc. Stockholm Power Tech Int. Symp. Elect. Power Eng., vol. 6, New York, NY, USA, 1995.
[3] M. Leijon, L. Gertmar, H. Frank, T. Karlsson, B. Johansson, K. Isaksson,U. wollstrom, and J. Martinsson, “Breaking conventions in electrical power plants,” in Proc. Int. Conf. Large High Voltage Elect. Syst CIGRE, Paris, France, 1998.
[4] M. Leijon, “Powerformer™ -a radically new rotating machine,” ABB 0Rev., pp. 21–6, 1998.

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